Glass solders are conventionally used for joining materials. The glass solders are predominantly introduced between the materials to be joined in powdered form, possibly with the aid of suitable adjuvants such as oils or polymer solutions. This arrangement of the materials to be joined and the powdered glass is then heated to a temperature which permits sintering of the glass particles. At this juncture, compaction of the powder by viscous flow takes place. A temperature is required which is higher than the dilatometric softening temperature. A problem with this is that no glass solders are available for joining temperatures below 400° C. which are at the same time corrosion-stable and have correspondingly high crystallization stability in order to permit the joining process to take place.
Possible compositions for such ultra-low sintering glass solders are primarily phosphate glasses and borate glasses. In both glass systems, glasses are produced with correspondingly low softening points. However, these glasses then contain considerable concentrations of B2O3 or P2O5 and alkali oxides and have only a low level of chemical stability. Therefore, they are unsuitable for many applications owing to the lack of corrosion stability. In this case, stability is considered, according to the application, to be, for example, the hydraulic stability or moisture stability in particular at 85° C. and 85% relative humidity for a period of 1000 h.
In addition, corresponding glasses usually have a high tendency to crystallization and so crystallization often takes place more quickly than compaction by viscous flow. Crystallization of the glass solder must never occur prior to complete sintering because otherwise the sintering process will come to a halt. However, for many applications, in particular in optical technologies, it is also disadvantageous when crystallization occurs after complete compaction since in that case, light scattering can be observed on the crystals. For many applications there is a maximum permissible temperature, which is determined, for example, in the case of semiconductors to be joined, by diffusion of doping substances.
A further problem is the adhesion of the glass solders to the materials to be joined; in that case, good wetting of these materials is always a favorable precondition.
In principle, tellurite glasses have favorable preconditions for many applications as joining glass. At this juncture, particular mention should be made of the possibility of setting glass transition temperatures of 300° C. and also somewhat lower.
Tellurite glasses also have sufficient corrosion stability for most applications. However, the crystallization tendency of tellurite glasses is insufficient for many applications or joining processes. For example, although the glass transition temperatures and the softening temperatures fall when tellurite glasses contain alkali oxide and/or zinc oxide, the crystallization tendency also increases thereby.